I have the same problem! I seem to overcome this by turning off all running programs, and pausing a few seconds before turning the unit off. It seems to have a problem if I remove it from the cradle and power off right away. Try leaving it till the display dims at least before turing off. The problem is intermittent. Try turning off the WiFi before powering off.
Annoying isn't it, particulary as it will not recharge from a USB connection when the battery is dead. or even work at all on USB with a dead battery without the charger.